__init__: add __version__ variable
__init__: add __version__ variable

This variable is described at https://www.python.org/dev/peps/pep-0396/

This will allow consumers of this module to determine the version number
at runtime. For example:

  import tendrl.node_agent
  print(tendrl.node_agent.__version__)

Tendrl/node-agent,r0h4n/node-agent,r0h4n/node-agent,Tendrl/node_agent,Tendrl/node-agent,Tendrl/node-agent,Tendrl/node_agent,r0h4n/node-agent